The present invention relates to a method of liquefying a stream that is enriched in methane. This stream is obtained from natural gas, and the product obtained by the method is referred to as liquefied natural gas (LNG).
In the article xe2x80x98Liquefaction cycle developmentsxe2x80x99 by R. Klein Nagelvoort, I Poll and A J Ooms, published in the proceedings of the 9th LNG International Conference, Nice, France, 17-20 October 1989 such a method is described.
The known method of liquefying a stream enriched in methane comprises the steps of:
a) supplying a natural gas stream at elevated pressure to a scrub column, removing in the scrub column heavier hydrocarbons from the natural gas stream which are withdrawn from the bottom of the scrub column to obtain a gaseous overhead stream withdrawn from the top of the scrub column, partly condensing the gaseous over head stream and removing from it a condensate stream to obtain the stream enriched in methane at elevated pressure;
b) liquefying the stream enriched in methane at elevated pressure in a tube arranged in a main heat exchanger by indirect heat exchange with a multicomponent refrigerant evaporating at low refrigerant pressure in the shell side of the main heat exchanger; and
c) compressing the multicomponent refrigerant withdrawn from the shell side of the main heat exchanger and partly condensing it at elevated refrigerant pressure in a tube arranged in an auxiliary heat exchanger by indirect heat exchange with an auxiliary multicomponent refrigerant evaporating at low auxiliary refrigerant pressure in the shell side of the auxiliary heat exchanger to obtain multicomponent refrigerant for use in step b).
In the scrub column the gas stream is contacted with liquid reflux, which has a lower temperature so as to further cool the gas stream. As a result heavier hydrocarbons of the gas stream are condensed and the formed liquid is collected in the bottom of the scrub column from where it is withdrawn.
In the known method, the liquid heavier hydrocarbons withdrawn from the bottom of the scrub column and the condensate stream from the gaseous overhead stream are passed to a fractionation unit to be partially condensed. From the fractionation column a stream is removed which is used as reflux in the scrub column.
Prior to supplying the natural gas stream in step a) to the scrub column, it is cooled. The temperature of the reflux stream should be significantly lower than that of the natural gas stream supplied to the scrub column. This requirement sets a lower limit for the temperature of the natural gas stream supplied to the scrub column.
In the known method, the natural gas stream is cooled in a tube arranged in the auxiliary heat exchanger before it is introduced into the scrub column. Thus the temperature of the cold end of the auxiliary heat exchanger is limited by the temperature of the reflux stream. Thus more heat has to be extracted in the main heat exchanger to liquefy the stream enriched in methane.
It is an object of the present invention to allow a lower temperature at the cold end of the auxiliary heat exchanger so that the amount of heat that is to be extracted in order to liquefy the stream enriched in methane is reduced.
To this end the method of liquefying a stream enriched in methane according to the present invention is characterized in that partly condensing the gaseous overhead stream is done in a tube arranged in the auxiliary heat exchanger.
In this may the temperature of the cold end of the auxiliary heat exchanger can be selected as low as practicable.
In the known method, the temperture of the multicomponent refrigerant withdrawn from the cold end of the auxiliary heat exchanger was also limited by the temperature of the reflux. An advantage of the method of the present invention is that this limitation has been removed. Consequently a lower circulation rate of the multicomponent refrigerant is required.
The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein
FIG. 1 shows schematically a flow scheme of the plant in which the method of the invention is carried out, and
FIG. 2 shows an alternative way of partly condensing the multicomponent refrigerant.